There are now a number of products on the market specifically designed for cleaning concrete, and when I tested them on the TV show, most of them appeared to work on asphalt stains. One of the keys is to brush in the cleaner, and then to use a high-pressure water power washer system to flush everything out of the pores of the concrete. You can purchase inexpensive power hoses, or rent them.
One of the products most friendly to the environment that I have found for removing all kinds of oil based stains from concrete and concrete blocks is a product called OIL LIFT. This is a completely bio-degradable product that actually goes in and lifts the oil out of the concrete and then breaks the hydro-carbon chains to make the oil based splashes bio-degradable as well. You can find it in Canadian Tire.